1. Field of the Invention
This invention relates to a trimming resistor in a thin film or thick film integrated circuit, and more particularly to a trimming resistor network used for an output characteristic controlling device such as a digital converter.
2. Description of the Related Art
Recently, in the semiconductor integrated circuit and hybrid integrated circuit, the functional trimming has received much attention as a means for attaining precise output characteristics.
Since laser beam trimming is effected by use of a laser beam, it is not necessary to set the trimming device in electrical contact with trimmed material. Therefore, in a case where a resistor is used as the main factor for determining the output characteristic of the circuit, the resistance of the resistor is first set to a proper initial value and then the resistor is cut or processed by use of a laser beam while the circuit is set in the operative condition and the output characteristic is being observed. Thus, the resistance of the resistor can be adjusted until a desired output characteristic can be attained and therefore a high precision of output characteristic can be obtained. This resistance adjusting method is called the functional trimming.
Various trimming methods for adjusting the resistance have been proposed. The following two main methods are practiced. One of the trimming methods is to use short bars 2 respectively connected is parallel with series-connected diffusion resistor elements or thin film resistor elements 1 as shown in the circuit of FIG. 7A. Short bars 2 are sequentially cut apart (indicated by mark x) to adjust the resistance between two terminals A and B. The other trimming method is shown in FIG. 7B. FIG. 7B is a plan view of thin film resistor 4, and reference numeral 3 denotes a metal electrode. With this method, the resistance can be adjusted by forming groove 5 in the resistance film to change the direction of electric lines of force in the resistor film.
Now, the problem in the conventional technique is explained with reference to the functional trimming used in a semiconductor integrated circuit as an example.
In FIG. 7A, short bars 2 are generally formed of electrode wiring metal material such as Al. Since the metal material has high heat conductivity and a large light reflection factor, a large power laser beam is necessary when the laser beam is applied to the short bars to heat, melt and cut the same. In this case, if the short bars are arranged in a region of the semiconductor integrated circuit, the laser beam will be applied to the layer lying under the short bar immediately after the short bars are cut off by the laser trimming operation. Therefore, the underlying oxide film and semiconductor substrate may be destroyed. Further, the smallest variation in the surface condition of the metal material disposed in the short bar forming step will change the reflection factor thereof. In this case, the condition of application of the laser beam power required for cutting the short bar is changed and it is extremely difficult to effect the proper trimming without destroying the underlying layer.
In contrast, in the groove formation method of FIG. 7B, it is possible to process thin film resistor 4 without destroying the underlying layer by forming thin film resistor 4 of material such as polysilicon which has lower heat conductivity than metal. However, a small crack (called microcrack) 6 is formed in the processed fracture portion. The microcrack grows with heat or mechanical stress or absorbs moisture, causing variation in the resistance with time. The variation in the resistance with time is a fatal defect for a circuit used for adjusting the precise output characteristic by functional trimming.
In order to prevent variation in the resistance with time, there is provided a method in which thin film resistors are selectively cut off so as to prevent the electric line of force from crossing the fracture portion, to thereby adjust the resistance. In this case, if the trimming film resistors of the same resistance are connected in parallel, the amount of variation in the resistance for each cutting operation is not constant. For example, when 10 film resistors of 10 .OMEGA. are connected in parallel as shown in FIG. 8, the initial resistance between terminals A and B is 1 .OMEGA.. When one of the film resistors is cut off, the resistance increases by 0.1 .OMEGA.. However, when one of the two remaining film resistors is cut off, the resistance between terminals A and B changes from 5 .OMEGA. to 10 .OMEGA., and thus the amount of variation is 5 .OMEGA.. That is, in this method, it is difficult to change the resistance by a desired amount as required.
Even with the above parallel circuit, it is possible to make the amount of resistance variation for each cutting operation constant. FIG. 9 shows an example of a network with which the resistance can be varied by 1 .OMEGA. for each cutting operation so as to change the resistance from 1 .OMEGA. to 10 .OMEGA.. The initial value of the combined resistance between terminals A and B is 1 .OMEGA.. The resultant resistance can be changed by 1 .OMEGA. by sequentially cutting off a resistor in a direction from the left to the right in the drawing so that the combined resistance can be changed from 1 .OMEGA. to 10 .OMEGA.. With this method, variation in resistance with time due to the presence of a microcrack will not occur. However, it is necessary to occupy a large area in order to form a film resistor having resistances of 2 .OMEGA. to 90 .OMEGA.. As a result, the cost increases because of increase in the chip area of the semiconductor integrated circuit, making it impossible to perform the method practically.
The conventional methods for changing the resistance of the trimmed resistor have the following problem. In the method described in FIG. 7A in which the short bar of good heat conductivity is cut off, the underlying layer may be damaged. Further, in the method of FIG. 7B in which a groove is formed in the resistor film, resistance variation wit time due to the presence of a microcrack may occur.
Further, in the method of FIG. 8 in which film resistors are connected in parallel and are sequentially cut off, the problem of microcracks can be solved but the amount of resistance variation unit in each cutting operation cannot be made constant. In this case, the resistance variation amount in each cutting operation may sometimes significantly exceed a variation amount required for attaining a desired output characteristic of the integrated circuit. In the method of FIG. 9, a constant resistance variation unit required for attaining a desired output characteristic can be obtained. In this case, however, the difference between desired resistance values of the film resistors constituting the network is large, and a large area is required for formation of the film resistors, making this method impractical.